The present invention relates to a spinal column fixation device, and in particular to a device of this type used in the treatment of scoliosis. Scoliosis is a three dimensional curvature of the spine in which the spine, when viewed from the posterior aspect, has an abnormal lateral curvature. In additional to the lateral curvature, there is a curvature in a plane perpendicular to the axis of the spine, so that the combined curvature is somewhat spiral in nature. Scoliosis is much more prevalent in young children and adolescents than in adults, and preferentially afflicts females.
For scoliosis in an adolescent, the normal procedure of choice is the insertion of spinal instrumentation to correct the curvature immediately followed by a fusion of the spine. After the curvature has been corrected by manipulation of the metal instrumentation, the facet joints on either side of the spine in the area that is affected by the curvature are then removed and small strips of bone taken from the illiac crest area of the pelvis, for example, are laid along the posterior elements out to the tips of the transverse processes. The metal instrumentation remains attached to the spinal column to stabilize the spinal column thereby permitting the inserted strips of bone to fuse into a solid mass with the spinal column. Normally the metal instrumentation is never removed, but after the definitive fusion has taken place, the instrumentation is somewhat superfluous.
The most common type of instrumentation utilized for this procedure is the Harrington distraction and compression system wherein separate hooks, one above and one below the fusion mass, are hooked around the laminae and are connected together by a solid rod having ratchets on the upper end thereof. By manipulation of the ratchet mechanism, the hooks are spread apart, thereby pulling the curvature out of the spinal column. If compression is also utilized, lateral hooks engage the transverse processes on the convex side of the curvature and are then pulled toward the distraction rod to assist in straightening the spinal column.
A disadvantage to the Harrington system is that it is not inherently a strong system for continued fixation of the spinal column. Since there are only two points of fixation for the exertion of distraction forces, if one of the hooks fails or the rod breaks, which sometimes occurs in the areas of the ratchets, the spinal column is totally destabilized and will assume its previous scoliotic curvature if the spinal fusion has not yet completely formed. Even simple day to day activities place a tremendous amount of stress on the spinal column, and it is not unusual for metal instrumentation, such as that used in the Harrington system, to fail.
Subsequently, the Luque fixation system was developed. This system comprises two L-shaped solid rods which are wired together in the form of a rectangle with the upper cross member extending across the posterior side of the spinal column above the area of the abnormal curvature, and the lower cross member for the other, inverted rod, passing across the spinal column at a point below the curvature. This rectangular-shaped appliance is positioned against the posterior side of the spine such that the elongate rods are located on each side of the spinous processes, lying between the spinous and respective transverse processes. The rods are fixed to the spinal column by means of individual wires which are passed around the rods and underneath the posterior lamina.
The advantage of the Luque system is that it is a much stronger system than the Harrington system, primarily because of the fact that it is fixed at each segment of the spine, so that if one or two wires were to break, total fixation of the spine would not be lost. A further advantage to the Luque system is that it is strong in the direction of forces within the plane of the rectangle defined by the two L-shaped rods.
One of the disadvantages of the double L-rod Luque system is that the two elements making up the rectangle are not rigidly fixed at the corners of attachment. Accordingly, some strength against torsional forces is lost. Many of the activities that a person is involved in require twisting of the spine, so that if two much twisting is permitted, acceptable fusion of the spine may be delayed or prevented altogether. Furthermore, the potential for breaking the wires is much greater if the patient engages in activities which produce a high degree of torsional movement.
A further development to the Luque system is the provision of a solid rectangle, which results in much better stability against torsional forces.
A further development of the Luque system has resulted in a solid rectangular appliance. Although this appliance provides much better stability from the standpoint of torsional forces, the appliance must be manufactured to fit exactly the size of spine to which it will be fitted. Because there is substantial variation in the length of the human spine, it is often necessary to stock a large number of these appliances. Also, it may not be until the patient has been opened for implanting of the appliance that it is discovered the appliance on hand will not fit.
A small child, such as one below the age of 9 or 10, that has severe scoliosis presents a particularly troublesome problem that is not present in an older child or adolescent. Particularly if the child has scoliosis which is progressive, external type braces will often not be sufficient to arrest progression of the condition, and a surgical procedure is necessary.
Both the Harrington and Luque systems have been utilized in fixation of the spines of young children, but if the fusion is done at the time of initial fixation, the spine cannot grow any further and since the rest of the body will grow somewhat normally, the child ends up with an abnormally and unproportionally short trunk. From a cosmetic standpoint, this is extremely undesirable, although it is often necessary with rapidly progressing scoliosis.
If the spine is not fused at the time of initial instrumentation with the Harrington rods, the growth of the spine will progressively loosen and reduce the effectiveness of the distraction rod, so that by the time the child is ready for the definitive fusion procedure, the spine will have regained some of its original scoliotic curve, or will require repeated tightening of the Harrington rod.
Although the original double-L Luque system on a small child to fix the spine until the spine has developed to the point where the fusion procedure can be done results in stronger fixation of the spine than does the Harrington system, the two L-shaped rods tend to be pulled apart by growth of the spine, because the bars must be wired on the vertebrae sufficiently loose to enable movement of the individual vertebrae relative to each other. It has been observed that, after the passage of three or four years, the wires which previously attached the individual vertebrae to the rods are likely to have slipped off the ends of the rods. When this occurs, there is no longer any fixation of the spine and it is then able to reassume its previous scoliotic curve.
In the case of the solid rectangular Luque system, or where the double-L rod Luque system has the rods very tightly wired together so that relative sliding between the two rods is not possible, the child's spinal column will not be able to grow because it is fixed against movement in the longitudinal direction. This will result in a shortened spine and trunk just as would occur if the spine were fused at the time the instrumentation was implanted; or the wires will break under the strong forces of growth.